Sodium balance, arterial pressure, and the role of the subfornical organ during chronic changes in dietary salt

The subfornical organ (SFO), one of the brain circumventricular organs, is known to mediate some of the central effects of angiotensin II related to sodium and water homeostasis. Because angiotensin II levels are altered with changes in chronic dietary salt intake, we reasoned that the actions of angiotensin II at the SFO might be involved in the regulation of arterial pressure during long-term alterations in dietary salt. The present study was designed to test the hypothesis that long-term control of arterial pressure during chronic changes in dietary salt intake requires an intact SFO. Male Sprague-Dawley rats were randomly selected for electrolytic lesion (SFOx, n = 8) or sham (n = 9) operation of the SFO. After a 1-wk recovery period, rats were instrumented with radio-telemetric blood pressure transducers for continuous 24-h measurement of mean arterial pressure (MAP) and heart rate (HR) and then were placed individually in metabolic cages. After another 1 wk of recovery, the rats were subjected to a 49-day protocol as follows: 1) a 7-day control period (1.0% NaCl diet), 2) 14 days of high-salt (4.0% NaCl) diet, 3) 7 days of normal-salt (1.0% NaCl) diet, 4) 14 days of low-salt (0.1% NaCl) diet, and 5) 7 days of recovery (1.0% NaCl diet). There were no significant differences in MAP or HR between SFOx and sham-operated rats throughout the protocol. These results do not support the hypothesis that the SFO is necessary for regulation of arterial pressure during chronic changes in dietary salt. However, SFOx rats demonstrated significantly less cumulative sodium balance than sham-operated rats on days 2-6 of the high-salt diet period. These data suggest that the SFO is important in the regulation of sodium homeostasis during chronic changes in salt intake.